For various pyrotechnic requirements and especially to ensure correct inflation of the protective cushions, pyrotechnic gas generators must supply, within extremely short periods of the order of thirty milliseconds, gases which are clean, that is to say free from solid particles capable of constituting hot spots that could damage the wall of the cushion, and which are nontoxic, that is to say with low contents of nitrogen oxides, carbon oxides and chlorine-containing products.
Various classes of pyrotechnic compositions have been developed with this aim in mind.
A first class relates to the compositions based on an alkali-metal or alkaline-earth metal azide in the presence of an inorganic oxidizing agent such as potassium nitrate or a metal oxide. These compositions which may, if appropriate, comprise a binder have two major disadvantages. On the one hand, when they are burning they produce much dust which must be filtered out by relatively large filtering systems, and this increases the weight and the price of the generator at the same time. On the other hand, azides are highly moisture-sensitive materials and these compositions are difficult to store in good conditions for several years in a motor vehicle.
A second class relates to the compositions based on nitrocellulose and nitroglycerine. These compositions, also known by the name of "double-base powders" are very advantageous because they burn very fast and without producing dust. However, they nevertheless have the disadvantage of not being completely stable with time, because of the phenomenon of migration of the nitroglycerine, a phenomenon that, as years go by, impairs the effectiveness of these compositions in a motor vehicle.
A third class relates to so-called "plastic-bonded" compositions consisting basically of an organic binder and of an oxidizing inorganic filler such as, especially, an inorganic perchlorate. These compositions are a priori very advantageous because they have a high rate of burning and an excellent stability on aging.
Thus, Patent FR-A-2 137 619 or its equivalent U.S. Pat. No. 3,723,205 proposes compositions in which the binder is a polyvinyl chloride and in which the oxidizing filler is an ammonium perchlorate in the presence of sodium nitrate as internal chlorine-scavenger. Nevertheless, the use of a chlorine-containing binder in the presence of energetic filler is tricky to implement, especially where safety is concerned.
Plastic-bonded compositions consisting of a silicone binder capable of crosslinking at ambient temperature, also known by the name of "RTV" (Room Temperature Vulcanizable) binder and of potassium perchlorate were then proposed, the potassium atom acting as an internal chlorine-scavenger. Such compositions are, for example, described in Patents FR-A-2 190 776 and FR-B-2 213 254 or in their American equivalents U.S. Pat. Nos. 3,986,908 and 3,964,256.
More recently, plastic-bonded compositions consisting of a silicone binder of the RTV type and of fillers of the metal azide type or nitro compounds such as tetrazole have also been proposed in patent application WO94/06735.
The use of plastic-bonded compositions with a silicone binder is actually very advantageous in the field of motor vehicle safety, insofar as the silicone matrix, when burning, produces a glassy residue which ensures a first filtration of the combustion gases. Nevertheless, compositions of this type have the disadvantage of being tricky to use if the intention is to employ continuous manufacturing processes making use of an extrusion stage, because of the poor mechanical strength of uncrosslinked silicone binders.
From the teaching of Patent U.S. Pat. No. 3,367,816 it is known to extrude continuously an uncrosslinked silicone resin dough to ensure the internal coating of continuously manufactured pipes. However, at the present time it is not known to extrude continuously objects with an uncrosslinked silicone matrix which has a form and dimensions that are fully defined and stable, so as to be capable of being subsequently crosslinked without any change in the geometrical parameters.
At the present time the pyrotechnic compositions with a silicone binder are therefore employed in the form of granulates and not in the form of a single-block charge that has a shape and dimensions which are fully defined, because such charges cannot be obtained by continuous processes but only by noncontinuous processes for forming and curing in a mould, which are prohibitively expensive for the motor vehicle industry.
To ensure the reliability of the operation of the pyrotechnic generator which is subjected to many mechanical stresses, however, it is clearly preferable that it should contain a charge of pyrotechnic composition in the form of a block whose geometry is relatively insensitive to mechanical stresses and especially to repeated vibrations, rather than a bulk granulate charge.
The person skilled in the art is therefore searching for a continuous process which would enable him or her to obtain plastic-bonded pyrotechnic charges with a silicone binder which have a defined and reliable geometry, especially in the case where the said charge must have small thicknesses to be burnt.